JP4793052B2 - High-strength steel sheet excellent in phosphate treatment and method for producing the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a high-strength steel plate excellent in phosphatability optimum for a steel plate for automobiles, particularly a high-strength steel plate having a high Si content, and a method for producing the same.

  In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of global environmental conservation. In addition, from the viewpoint of occupant protection in the event of a collision, it is also required to improve the safety of the automobile body. For this reason, the application of high-strength steel sheets to automobile parts has been actively promoted for the purpose of reducing the weight and strengthening of automobile bodies.

  On the other hand, steel plates used as automotive parts are generally subjected to phosphate treatment as a coating ground treatment after press forming. However, in high-strength steel sheets, inexpensive Si is often used as a reinforcing element, and one of the major problems is that it is inferior in phosphatability compared with soft steel sheets.

  Even in general soft steel sheets, it is known that the phosphate processability deteriorates when cooling during continuous annealing is performed by water cooling or air-water cooling, etc. Many studies have been made as listed below.

Patent Documents 1 to 10 describe a method of attaching a metal element other than Fe, for example, Ni, Co, Ti, Mn, Cu, Mo, W, etc., to the steel sheet surface. Patent Document 12 discloses a method of performing P plating, Patent Document 12 discloses a method of cooling a steel sheet with a coolant containing an organic acid, an inorganic acid, or a salt thereof, and Patent Document 13 discloses a method of performing boning on a steel sheet after completion of cooling during continuous annealing. A method of contacting an acid aqueous solution is disclosed.
JP 58-55535 A Japanese Unexamined Patent Publication No. 56-116883 JP 58-66666 A JP 59-159987 A JP-A-61-149492 JP 61-23794 JP-A-3-75382 JP-A-3-86302 Japanese Patent Laid-Open No. 3-126879 Japanese Unexamined Patent Publication No. 7-278843 JP-A-61-136694 Japanese Unexamined Patent Publication No. 1-139728 JP-A-2-70969

  However, each of the above methods is effective for improving the phosphate processability in a general soft steel sheet having a low Si content, but is sufficiently effective for a high strength steel sheet having a high Si content. It was not right.

  An object of the present invention is to provide a high-strength steel sheet and a method for producing the same, which can obtain excellent phosphate processability even when the Si content is high.

  Conventionally, it is considered that the phosphate treatment property of high-strength steel sheets having a high Si content is inferior due to Si-based oxides generated on the steel sheet surface during annealing. In fact, the present inventors conducted high-strength steel sheets with a high Si content and SEM-observed the steel sheet surfaces before and after phosphating at a low acceleration voltage of 500 V with the same field of view. It was proved that the formation of phosphate crystals was inhibited and the phosphate treatment ability was deteriorated. Therefore, when a method for effectively removing such Si-based oxides was examined, it was found that pickling with hydrofluoric acid was effective.

  The present invention has been made on the basis of such findings, and is characterized in that the surface of a steel plate after annealing is pickled with hydrofluoric acid or an acid obtained by mixing hydrofluoric acid and another acid. The manufacturing method of the high-strength steel plate excellent in and the high-strength steel plate manufactured by the method are provided.

  It is preferable to carry out Ni, Ni—P or Zn—O plating after pickling with hydrofluoric acid or an acid mixed with hydrofluoric acid and another acid.

  According to the present invention, even when the Si content is high, a high-strength steel sheet capable of obtaining an excellent phosphatability can be produced. Moreover, since cheap Si can be used for this invention, it is very useful also for the cost reduction of a high strength steel plate.

  As described above, the present inventors have demonstrated that the phosphate treatment property of the high-strength steel sheet having a high Si content is inferior to the Si-based oxide generated on the surface of the steel sheet during annealing. It has been found that this Si-based oxide can be effectively removed by pickling with an iron. The details will be described below.

  Si-based oxides formed on the surface of the steel sheet can be removed by pickling with hydrochloric acid or sulfuric acid, but they need to be pickled for a very long time of about several minutes and are not practical. Nitric acid mixed with nitric acid and hydrochloric acid can remove Si-based oxides efficiently, but nitric acid etches the steel sheet vigorously, so that the steel sheet surface is roughened and the surface appearance is remarkably impaired.

  On the other hand, in the pickling with hydrofluoric acid according to the present invention, the Si-based oxide can be removed by a short pickling without etching the steel sheet. The concentration of hydrofluoric acid, pickling temperature, pickling time, etc. are not particularly limited. For example, when pickling with a mixed solution of pure water 4: hydrofluoric acid (50%) 1, most of the content of Si is high. The Si-based oxides could be completely removed by pickling for about 10 s at room temperature on high-strength steel sheets. Note that hydrofluoric acid is inferior in the effect of removing organic matter, so when degradation of phosphate treatment due to organic stains becomes a problem, hydrofluoric acid and other acids such as hydrochloric acid, sulfuric acid, nitric acid are mixed. Need to be pickled with acid. The concentration of the acid to be mixed is not particularly limited. For example, an acid such as hydrochloric acid, sulfuric acid, and nitric acid may be mixed in a hydrofluoric acid aqueous solution at a concentration of 100 g / liter. Further, the same effect can be obtained even when pickling continuously with hydrofluoric acid and other acids.

  The amount of Si-based oxide on the surface of the steel sheet can be evaluated by the Si oxide coverage determined as follows using a scanning electron microscope. In other words, the inventors used a Carl Zeiss scanning electron microscope (Supra55VP), with an acceleration voltage of 0.5 kV, a working distance of 4 mm, and the use of an in-lens detector, with a direct magnification of 500 times (field of view approximately 20 μm × 15 μm). 5), the surface area of the steel sheet was photographed in five fields of view, and the area ratio of the place that was visually judged as Si oxide (observed with black contrast) was defined as the Si oxide coverage.

  Depending on the high-strength steel sheet having a high Si content, pickling with hydrofluoric acid or an acid mixed with hydrofluoric acid and other acids may cause precipitation and impurities on the surface of the steel sheet that form the core of phosphate crystal formation. In some cases, the phosphate crystal becomes coarse and the phosphate crystal becomes coarse, so that uniform phosphatability on the steel sheet surface cannot be obtained. In such a case, phosphate crystals can be prevented from becoming coarse by performing pickling with hydrofluoric acid or an acid mixed with hydrofluoric acid and another acid, followed by Ni, Ni-P or Zn-O plating.

  In steelworks, cold-rolled steel sheets A to C with the components shown in Table 1 manufactured by direct-fired heating type continuous annealing are pickled at room temperature for 10 s using hydrochloric acid, sulfuric acid, nitric hydrochloric acid and hydrofluoric acid. Then, phosphating was performed to prepare Sample Nos. 1 to 12. At this time, each acid used was diluted with pure water, and each concentration was hydrochloric acid: 278 g / liter, sulfuric acid: 125 g / liter, nitric hydrochloric acid: 165 g / liter nitric acid + 6.5 g / liter hydrochloric acid, hydrofluoric acid: Pure water 4: hydrofluoric acid (50%) 1 The phosphate treatment was carried out using a Surf Dyne 2800HM system manufactured by Nippon Paint Co., Ltd. according to the building bath and treatment conditions specified in this system. The degreasing time was 120 s for spraying, 40 s for washing with water, 30 s for surface conditioning, and 120 ° C. for chemical conversion treatment. Here, the total acidity of the chemical conversion treatment liquid was 25.1, the free acidity was 0.9, the toner value was 3.0, and the bath temperature was 43 ° C.

And the coverage of Si oxide was measured by said method. The phosphate treatment was evaluated as follows by visual observation and scanning electron microscope (hereinafter referred to as SEM) observation. In the visual evaluation, those having a uniform appearance were marked with ◯, and those having a non-uniform appearance were marked with ×. In the SEM evaluation, a fine phosphate crystal densely coats the underlying steel sheet, ○, there is a phosphate crystal with a particle size of 10 μm or more, and there is a slight scale (part where the underlying steel sheet is recognized). Some were marked with Δ, and those with a noticeable scale were marked with ×.
Visual evaluation: ○, SEM evaluation: ○
In this case, it was assumed that the phosphatability was good.

  Table 2 shows the results. Samples Nos. 10 to 12 that were pickled with hydrofluoric acid according to the present invention have a low Si oxide coverage, and show good phosphate treatment properties in both visual evaluation and SEM evaluation.

In addition, when cold-rolled steel sheets having the components of cold-rolled steel sheet A shown in Table 1 were manufactured by all-radiant tube type continuous annealing and examined for phosphate treatment as described above, pickled with hydrofluoric acid. Even SEM evaluation was Δ. In this case, the SEM evaluation could be made good by applying Ni plating, Ni-P plating or Zn-O plating with an adhesion amount of 20 mg / m ² after pickling.

Claims (3)

  A method for producing a high-strength steel sheet excellent in phosphate treatability, characterized by pickling the surface of a steel sheet after annealing with hydrofluoric acid or an acid obtained by mixing hydrofluoric acid and another acid.   2. The phosphate treatment ability is excellent according to claim 1, characterized in that after pickling with hydrofluoric acid or an acid mixed with hydrofluoric acid and another acid, Ni, Ni-P or Zn-O plating is performed. Manufacturing method of high strength steel sheet.   A high-strength steel sheet excellent in phosphate processability manufactured by the manufacturing method according to claim 1 or 2.